BFUe, the most primitive committed erythroid progenitors, are capable of extensive proliferation. BFUe differentiate into CFUe, cells with very restricted proliferative potential. Pu.1 is a hematopoietic, master regulatory ETS transcription factor (TF) that is required for BFUe self-renewal. PU.1 inhibits BFUe differentiation. Down regulation of PU.1 occurs and is required for BFUe to differentiate into CFUe. Despite Pu.1's central role in regulating the terminal differentiation decision, the molecular mechanism controlling Pu.1's expression and down-regulation are not known. We propose to identify the cis-regulatory elements that control Pu.1 expression in BFUe and it's down regulation as these progenitors enter terminal differentiation. Using computational analysis of gene expression, ChIP Seq and other types of genome-wide data sets, we have identified several DNA binding proteins as candidates for controlling Pu.1 expression in BFUe, including SATB1, E2F4, Runx and Jun. The analysis also suggests that some of these factors, Runx and Jun, collaborate with Pu.1 to regulate a large set of genes that promote proliferation of BFUe and antagonize their terminal differentiation. We propose to determine whether these factors regulate Pu.1 expression. We also propose to determine whether they work in concert with Pu.1 to control a set of important gene targets in erythroid cells. The successful completion of this work will provide essential insights into the transcriptional networks that regulate red blood cell output.